The invention relates to a transmitter for a two-wire, differential databus on which a dominant state can be impressed by means of the transmitter in an active state of the transmitter, and which is in a recessive state when all transmitters connected to the databus are in a passive state.
Such transmitters, for example, for the so-called CAN databus, are characterized in that in their active state, in which they impress the dominant state on the databus, one line of the databus is set to a high level and the other line of the databus is set to a low level. When this is done simultaneously, there will be hardly any electromagnetic radiation because the radiation of one bus line then compensates the radiation of the other bus line with the reverse sign. However, actually, this ideal state cannot be achieved because particularly the transistors used for switching the signals to the databus are not ideal. This in turn has the result that, particularly when switching on and switching off the active state or the defined level at the two databus lines, there is electromagnetic radiation via the lines of the databus.
In some known transmitters, it is attempted to reduce the radiation by dividing the terminating resistors of the databus and by connecting the junction point between the divided terminating resistors to a reference potential. It is true that the radiation can thereby be reduced but it is still high enough to have a possibly disturbing effect.
It is an object of the invention to improve a transmitter of the type described in the opening paragraph in such a way that there is a further reduced electromagnetic radiation when switching the states.
This object is achieved in that the transmitter is provided with a capacitance and switching means by means of which the capacitance can be alternatively coupled to an electric source or between the two databus lines, and in that the switching means charge the capacitance by means of the electric source during periods when the transmitter is in a passive state, and couple the capacitance between the two databus lines during periods when the transmitter is in an active state.
In contrast to known transmitters, in which a voltage source is directly connected to the lines of the databus so as to impress the dominant state on the databus in the active state, a capacitance is used in the invention which, during the active state phases of the transmitter, is connected to the two databus lines. Now, it does not matter whether the two electronic switches, by means of which the capacitance is connected to the databus, have equal switching characteristics. In fact, at the instant when the capacitance is connected via the switching means to the two lines of the databus, the voltage on the two lines of the bus is exclusively defined by the current which flows through the capacitance and not by a potential of the voltage source; on the contrary, in the transmitter according to the invention, the capacitance has a floating potential. It is thereby achieved that there will be only a minimal electromagnetic radiation, even when the two terminals of the capacitance are not connected exactly simultaneously to the databus lines.
During the periods when the transmitter is in the passive state, the capacitance is charged by means of an electric source, for example a voltage or current source. Upon a transition from the passive state to the active state, the capacitance is switched off by the voltage source and instead connected to the two databus lines. Conversely, upon a transition from the active to the passive state, the capacitance is disconnected from the databus lines and instead connected to the electric source.
It is thereby achieved with very simple means to reduce the electromagnetic radiation to a strong extent when switching between the two states.
In an embodiment of the invention as defined in claim 2, the switching means are electronic switches, two of which are provided for connection to the voltage source and for connection to the two databus lines. Advantageously, transistors, preferably DMOS transistors as defined in claim 5 are used as electronic switches in a further embodiment of the invention. DMOS transistors have a low internal resistance in the active state so that switching of the capacitance alternatively to the voltage source or the databus lines can be effected with small contact resistances.
In the case where the transmitter operates incorrectly, it might occur that it is permanently attempted to transmit a dominant bit, which is actually not in accordance with the transmission standard. In this case, the capacitance would have a high capacitive load at the databus lines. To avoid this, two diodes as defined in claim 6 may be advantageously provided for such a disturbing case, which diodes prevent the capacitance at the databus lines from operating as a capacitive load.
When DMOS transistors are used as electronic switches, two further diodes may be advantageously provided as defined in claim 7, which diodes open current paths through the DMOS transistors in the case of a short circuit, thus in the case where one or both lines of the databus are short-circuited against reference potential. This is prevented by the two additional diodes as defined in claim 7.
In accordance with a further embodiment as defined in claim 8, a further DMOS transistor, which is arranged parallel to the DMOS transistor connecting the capacitance to that databus line which has a high level in the dominant state, is connected parallel to a fifth electronic switch (20) in the form of a DMOS transistor in such a way that this transistor can be turned on, also in the case of a short circuit of this line (1) of the databus. In the case of a short circuit of that databus line which has a high level in the dominant state, the DMOS transistor connecting this line to the capacitance can no longer be turned on under circumstances. The fifth transistor may be advantageously provided to be able to connect the line to the capacitance, also in such a disturbing case.
Since the electronic switches for switching the capacitance are, in principle, to be connected either to the voltage source or to the two databus lines in substantially similar states, an embodiment as defined in claim 9 advantageously provides only a control signal which is, however, to be delayed to a small extent for both groups of electronic switches.
The databus transmitter according to the invention may be advantageously provided particularly for a CAN bus (ISO 11898); all requirements imposed on the bus and on the properties of the transmitter are fulfilled, although the above-described small electromagnetic radiation is effected.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.